The present invention relates generally to Joule-Thomson cryostats, and more particularly, to an in-line valve flow controller for a Joule-Thomson cryostat.
The assignee of the present invention manufactures missile systems that use a focal plane array infrared sensor that requires cooling to approximately 90.degree. K. The temperature of the focal plane array and infrared sensor must be maintained within a few tenths of a degree Kelvin for precise target acquisition. Joule-Thomson coolers using argon or nitrogen gas are employed to cool the focal plane array infrared sensor.
Development of ultra fast cooldown cryostats by the assignee of the present invention has lead to a need for an actuator/flow controller that is compatible therewith. The flow controller is required to permit high flow during cooldown, and reduce flow once cooldown temperature is reached, while permitting a low flow to maintain the temperature of the focal plane array infrared sensor.
The Joule-Thomson valve flow controller currently used by the assignee of the present invention incorporates a needle as the valve stem. The needle is positioned off axis from the actuator stem, and therefore requires precise radial positioning to function. When the valve is open, the needle sits above the Joule-Thomson orifice. When actuated to close, the needle is draw into the orifice opening, blocking flow. Initial needle position and orifice condition dictate the open flow rate.
The needle must be positioned with great accuracy, with reliability and repeatability subject to the tolerances provided in manufacturing. The orifice opening is only about 0.006 inches in diameter, which is difficult to machine, and has been known to clog due to either contamination or wear from repeated usage. Bypass flow is accomplished through longitudinal scribe lines in the orifice seat, which in turn allows a small amount of flow past the needle. At best, this bypass flow arrangement is poorly repeatable owing to the difficulty in repeatably scribing a fine line in the seat. Also, after prolonged use, the scribe lines wear, and little or no bypass flow is obtained.
Accordingly, it is an objective of the present invention to provide for an improved Joule-Thomson cryostat valve flow controller.